The present invention relates to an audio reproduction system such as an audio preamplifier, an audio control amplifier, or the like, and more particularly to an improved tone control apparatus for use in such an audio reproduction system.
Generally, preamplifiers and control amplifiers for use in stereo reproduction systems have tone control circuits for altering the frequency response of the amplifiers thereby to adjust the tone of reproduced sound. Graphic equalizers are used as an independent circuit for fine adjustment of the frequency response of amplifiers connected thereto. Human ears suffer the natural loss of hearing sensitivity at lower and higher frequencies when the level of the reproduced sound level is low. Loudness control circuits are employed to compensate for such a tendency of human hearing by boosting or emphasizing high and low frequencies so that the reproduced sound heard by human ears has apparently flat frequency characteristics. The above tone control circuits, graphic equalizers, and loudness control circuits will be refered to as tone control units.
In any of the above tone controls, the adjusted frequency characteristics of an audio signal can be adjusted again manually by using a control knob or the like, but has nothing to do with changes in the volume level. Therefore, the adjusted frequency characteristics of an audio signal remain the same irrespective of whether the volume level of the signal is increased or reduced.
In designing an audio main amplifier, the gain of the main amplifier is selected such that the reproduced sound will not be distorted when the volume level is maximum, in order to ensure powerful impressions which the listener will have when the signal is reproduced with high output power. However, the reproduced sound will nevertheless be distorted if a signal in excess of an allowable maximum level is applied to the main amplifier. Such an excess signal is given, for example, when the volume level is increased while lower frequencies are being emphasized by a tone control unit. If the volume level is increased while lower frequencies are being accentuated, then only the bass portion of the frequency spectrum will be distorted. Some small-size stereo reproduction systems have a circuit for emphasizing low frequencies because the loudspeakers are too small to reproduce low frequencies. Car audio stereo systems also incorporate FIX equalizers, DSP circuits, or the like to achieve particular frequency characteristics for improving the acoustic properties of passenger compartments of cars. The amplifiers for use in those stereo systems must be set at a low gain level to prevent the reproduced sound at particular frequencies from being distorted. If the gains are selected in the usual manner, then the reproduced sound will be distorted at particular frequencies when the volume level is increased. The low-frequency distortion is particularly liable to occur with car audio stereo amplifiers whose gains are low and whose maximum output power is small.